The present invention relates to a front vehicle-body structure of a vehicle which comprises a pair of right-and-left front side frames extending substantially in a vehicle longitudinal direction at a front portion of the vehicle and a sub frame provided below the front side frames and supporting a pair of right-and-left suspension lower arms which constitute part of a vehicle suspension.
Conventionally, a structure in which collision energy occurring in a vehicle head-on collision is absorbed by deformation of front side frame is known. Further, a structure in which a sub frame supporting a vehicle suspension, a steering mechanism, a power train and others are provided below the front side frames is known. Herein, the sub frame comprises a sub-frame body portion and a pair of extension portions extending forward from the sub-frame body portion, which functions so as to absorb the collision energy secondarily.
In the structure equipped with the sub frame, it may be preferable that a rear end of the sub frame be detached from a vehicle body during a period from a middle stage to a final stage of the vehicle head-on collision in order not to block the smooth deformation of the front side frame. Accordingly, it is also known to provide a weak portion extending in a vehicle width direction at a specified position of the sub frame which is located just in front of a fastening portion to fasten the rear end of the sub frame, which can bend a portion near the rear end of the sub frame in the vehicle width direction, thereby pulling out a fastening member (bolt) (see Japanese Patent Laid-Open Publication No. 2011-162158).
Herein, it may be considered that the structure equipped with the extension portion of the sub frame for the secondary collision-energy absorption is combined with the structure disclosed in the above-described patent document, which is applied to a small type of vehicle. However, the small type of vehicle generally has a narrow vehicle width, so that an attachment point of a suspension lower arm constituting part of a vehicle suspension to the sub frame is located at an inward position in the vehicle width direction. Accordingly, it may be necessary to configure such that the above-described extension portion of the sub frame circumvents the suspension lower arm.
However, this may cause a problem in that the direction of a collision load occurring in the vehicle head-on collision which is transmitted from the extension portion to the sub-frame body portion slants so greatly inward relative to the vehicle longitudinal direction that a sufficiently-large load enough to bend the portion near the rear end of the sub frame in the vehicle width direction for proper detaching of the sub frame's rear end from the vehicle body could not be transmitted by way of the extension portion.
Moreover, in a case of the small type of vehicle having the narrow vehicle width, a light weight of the vehicle may be especially required by reducing the number of parts reduction or the like. Consequently, a space of the above-described fastening portion of the sub frame may be so limited that the above-described weak portion extending in the vehicle width direction could not be provided properly in some case.